1) Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for generating drawing data following a drawing instruction described in a page description language (PDL) or the like, and outputting an image drawn by using the generated drawing data.
2) Description of the Related Art
An image output apparatus such as a printer outputs an image onto a medium like a sheet of printing paper based on image information. This image output apparatus generates drawing data following a drawing instruction described in the PDL (which is usually transmitted from a host computer to a printer, in a printer system), and outputs the image onto the medium by using the generated drawing data. This image output apparatus receives the drawing instruction in the PDL format, and can carry out a graphic drawing based on the image information having a gray scale (i.e., gradation). Based on the graphic drawing instruction, the image output apparatus receives the instructions of a graphic range, concentration, and a filling of the graphic range, and can output a gradation image onto the medium.
As conventional examples of this graphic drawing that uses the gray scale, there are two Patent Literatures: Japanese Patent Application Laid-Open Nos. 7-168681 (hereinafter, “first patent literature”) and 2001-13954 (hereinafter, “second patent literature”).
The first patent literature titled “A PDL data processing apparatus, a control method therefor, and a pattern expanding apparatus” discloses an example of patterns A to D (refer to FIG. 29) that are used to draw an image, as an embodiment of a grayscale drawing. According to the first patent literature, when a new pattern appears, the old pattern data is deleted, and this new pattern is loaded. The first patent literature shows an example of loading two patterns (refer to FIG. 31), and a pattern count 3 deletes the pattern A to load the pattern C. The first patent literature assumes the drawing of an image under the condition that there occurs no significant problem when two patterns are loaded.
The first patent literature titled “A drawing apparatus, a drawing method, and a storage medium” discloses three image generators, including a mask generator, a background generator, and a gray generator. The gray generator generates a gray image based on object data. The gray generator writes gray image data into an address (which is determined based on the data generated from the mask generator and the background generator) at which the image is to be drawn on the image memory, based on a logic processing.
When the first patent literature is applied to the input drawing data like the photographic image having a mild gradation change, with different concentrations between adjacent pixels (i.e., different gradations corresponding to the number of pixels), the pattern data of the output image are loaded and deleted frequently. Consequently, the processing speed is reduced unavoidably. When there are pattern registers corresponding to the number of pixels, it is theoretically possible to avoid the reduction in the processing speed. However, this method is not realistic. Further, with the hardware disclosed in the first patent literature, it is difficult to hold the registers corresponding to the number of pixels, and this is not realistic from the viewpoint of cost. According to the second patent literature, only the gray data is input to the gray generator. Therefore, when the gray data changes corresponding to the number of pixels of the image such as the photographic image, the bit processing unit (BPU) operates by the number of times corresponding to the number of pixels, which hinders the increase in the processing speed.
The image output apparatus outputs a drawing image having a gradation, based on the graphic drawing instruction using the PDL. The apparatus follows the drawing instruction to fill a specified image drawing range in the page memory by using the specified concentration. According to the conventional practice, the image output apparatus processes each specified drawing range with the specified drawing range as a unit. Therefore, when there are various concentrations between adjacent drawing ranges (i.e., between pixels of the input drawing) like those of a photographic image that has a mild gradation change, each small drawing range has a specified concentration. The image output apparatus writes the bit map data using the concentration specified for each drawing range, into the page memory.
The image output apparatus usually writes the bit map data into the page memory by filling the drawing range with the data in a predetermined dither pattern (i.e., bedding in a tiled pattern) corresponding to the pixel concentration. Therefore, when the image output apparatus processes the photographic image, the apparatus repeats frequently a predetermined drawing processing, including a bit shifting of applying a dither pattern, to each small drawing range. This makes it difficult to increase the processing speed.
As explained above, according to the conventional methods of expanding the drawing data of a gradation image into the page memory based on the PDL graphic drawing instruction, it has been difficult to realize the high-speed processing when the method is applied to an image like a photographic image that has a mild gradation change.